Alternators and generators of conventional designs are mounted to an engine of an automotive vehicle to generate electricity and recharge the battery. The rotation of the rotor with respect to the stator generates electricity. The speed of the rotor is proportional to the speed of the engine because the alternator is belt driven by the engine.
One drawback to the rotating rotor is that at speeds of around 1,000 to 2,000 rpms, objectionable noise may be generated. Specifically, it has been found that at certain speeds associated with the engine idle speed (at about 1,000 to 3,000 rpms) a noise corresponding to a frequency 36 times the alternator speed, i.e., 36.sup.th order noise, is generated. The excessive 36.sup.th order noise in the idle speed range is due to the fact that 36.sup.th order magnetic forces, acting upon the stator teeth, induce certain types of vibrational modes in the alternator housing. These certain types of modes are highly efficient in producing noise. In addition to the 36.sup.th order forces, magnetic forces of other orders exist, and many other modes of vibration are excited. However, 36.sup.th order forces tend to excite modes that are very noisy, and this noise is most audible at idle, when the engine and other accessories are relatively quiet. It would therefore be desirable to eliminate or reduce the noise generated by the alternator.